Measurement of range, vibration, or speed of an object can be done without physical contact between a sensor and the object. Optical non-contact measurements usually involves transmitting light from a laser to the object and measuring changes in light scattered from the object by suitable detection means. Self-mixing laser interferometry, also known as laser-feedback interferometry and backscatter modulation interferometry, has been known for some time, and some of the characteristics of such sensors have been described previously. In comparison with traditional laser radar and laser Doppler velocimeters, typical previous devices are very simple, the entire optical assembly typically consisting of a laser and a lens to focus light on or near an object whose properties are being measured. The apparent simplicity of the hardware offers potential to manufacture sensors at a relatively low cost compared with the alternative techniques, in particular coherent (heterodyne) laser radar. In its simplest form the self-mixing sensor relies on the fact that if light at a substantially fixed frequency is sent from a laser to an object, and if a fraction of the light scattered from the object is permitted to re-enter the laser, interference causes a modulation of the laser power. The amplitude of the modulation is dependent on the amount of scattered power coupled back into the laser. The modulation is at least partly dependent on the phase of the backscattered light relative to the phase of the light circulating in the laser cavity. If the relative phase varies in time, as is the case if the scattered light is shifted in frequency due, e.g., to the Doppler effect, the result is a periodic variation of the laser power with time. Detection of the periodicity through spectral analysis provides a direct measurement of the Doppler frequency. Since the Doppler frequency shift f.sub.Doppler is related to the speed of the object v as f.sub.Doppler =2v/.lambda., where .lambda. is the laser wavelength, determination of the Doppler frequency directly leads to a determination of the object speed v. By introducing a known time-varying laser frequency shift, for example, where the transmitted laser frequency increases or decreases linearly with time, the range to a stationary or moving object can also be determined. In this case the time t between transmission and reception of light is given by t=2L/c, where L is the optical distance from the sensor to the target and c is the speed of light. If the laser frequency is varied at a rate df/dt, the sensor measures a frequency shift (df/dt) t if the object is stationary. If the object is moving, the detected frequency shift is (df/dt) t.+-.f.sub.Doppler, the + or - sign depending on whether the object is moving toward or away from the sensor, respectively. If the transmitted frequency is alternately moved up and down in a linear fashion, a measurement of the resulting two frequencies can permit determination of the range, speed, and direction of the object motion. Vibrations can also be detected using self-mixing sensors, since a vibration is simply motion with a velocity varying periodically in time. Length can also be measured by integrating the measured speed over time.
Early discussions of the self-mixing effect were given by D. M.Clunie and N. H.Rock (J. Sci. Instru., 41, pp. 489) in 1964 and M. J.Rudd (J. Sci. Instrum. Series 2, 1, pp.723) in 1968, where the researchers used a gas laser. Gas lasers were subsequently used also by Churnside (Appl. Optics 23, pp.61 and pp. 2097, 1984), Bearden and O'Neill (U.S. Pat. No. 5,260,562), and Hinz (U.S. Pat. No. 5,069,545). Such lasers are characterized by long laser cavities (typically on the order of 10 cm or more) and very high output coupler reflectivities (such as much greater than 90% in the case of HeNe lasers).
Self-mixing using semiconductor lasers (henceforth also called diode lasers) has been studied by several researchers. Shimizu (Appl. Optics 26, pp. 4541, 1987), Shinohara et al. (Appl. Optics, 25, pp. 1417, 1986), Meinzer et al. (U.S. Pat. No. 5,267,016), and deGroot et al. (Appl. Optics, 27, pp. 4475, 1988), describe various basic sensor aspects. DeGroot and Gallatin (Opt. Lett., 14, pp. 165, 1989) subsequently discussed the use of a long external cavity laser to enable measurements to a range of 50 m. As implemented, it is believed the laser required an optical spectrum analyzer and a feedback control system to keep the laser stable.
A previous patent to Gerardin (U.S. Pat. No. 4,928,152), as well as publications by Jentink et al. (Appl. Opt. 27, pp. 379, 1988), and Otsuka (Appl. Opt., 33, pp. 111, 1994) using a diode-pumped solid-state laser, improperly discuss the self-mixing effect as a conventional heterodyne mixing process. Wang et al. (J. Lightwave Techn., 12, pp. 1577, 1994) and Koelink et al. (Appl. Opt., 31, pp. 3401, 1992) note that at high feedback levels the laser can operate in multiple longitudinal modes.
Although the self-mixing phenomenon has been known for a relatively long time, several assumptions are usually made which limit the practical use of these sensors. For example, previously it was often assumed , that if a semiconductor diode laser is substituted in place of a gas laser, the sensor performance will not be strongly affected, in terms of sensitivity and dynamic range. Dynamic range refers to the ratio of the strongest signal that can be detected to the weakest detectable signal. We find that, with good design, the lower limit can approach the quantum limit. In this limit, the noise level is given by the expression hvB, where hv denotes the photon energy and B denotes the detection bandwidth. Previously, it was frequently (implicitly) assumed that the upper limit is determined by how much light can be collected and allowed to re-enter the laser cavity. This assumption is, however, incorrect. The upper limit is more frequently determined by the onset of external-cavity mode hopping, hereinafter referred to as EMH. If a laser is operating substantially on a single longitudinal mode (a single frequency), feedback perturbs the laser frequency, in addition to perturbing the laser power. If the feedback level is high enough and the phase of the returned light is changing with time, the laser may jump between different frequencies separated approximately by an integer number times the quantity c/2L, where L denotes the distance from the sensor to the target. In the case of a Fabry-Perot laser cavity, for reflectivity values below a limiting value R.sub.max given by the expression ##EQU1##
such EMH does not occur. Here L.sub.d denotes the optical length of the laser cavity, R.sub.oc denotes the laser output coupling mirror reflectivity, and .alpha. denotes the so-called linewidth enhancement parameter having a typical value in the range of 2-5. For external target reflectivity values in excess of R.sub.max, EMH becomes important and may have a significant influence on the accuracy of measurements. It is evident from the above equation 1, that as the distance L increases, the maximum allowable value of R.sub.max decreases. It is also evident that R.sub.max can be increased by increasing the laser length L.sub.d.
For gas lasers used at a modest distance, the combination of a large R.sub.oc and a large L.sub.d cause the upper limit on reflected light to be so high that the upper limit given by equation 1 is almost never encountered. Consequently, the EMH effect typically has not been discussed in the context of gas lasers. Furthermore, the EMH concept is believed unique to self-mixing sensors. Consequently, it typically has not been recognized by those authors who discuss the sensors as conventional heterodyne devices.
Among the prior authors who do note the existence of EMH, it is believed Wang makes no comments about whether the effect affects the sensor performance and shows no results to support any potential performance degradation. Koelink notes that under high feedback conditions, more noise results, and reducing the aperture can result in a larger signal without mention of attenuation effects or other possible rationales. It is believed no discussion is given of whether the issue is detrimental to unambiguous measurements, or how it can be circumvented.
In the one case where a long cavity diode laser has been fabricated, the device was put together in order to improve the coherence length of the laser source and hence extend the range of the sensor. The authors do not mention EMH, or it potential importance. The authors state that they used the long cavity to ensure a long coherence length of the source laser, but make no mention of the fact that a long laser may be more immune to high feedback levels. Having a sufficient coherence length is necessary, but not a sufficient requirement for operation at extended distances. In other words, it is believed a very short laser will never be highly suitable for very long range measurements, regardless of its coherence length.
It is believed the laser used by deGroot also does not enable low cost manufacturing. As discussed in the reference, the laser comprises a short diode laser together with an external mirror and control electronics to keep the laser properly aligned. Such lasers involve many precision components and are relatively expensive to fabricate.
Of fundamental importance is also that previously used diode laser devices are intrinsically unsuitable for sensor manufacturing. This is particularly true when the objective is to manufacture sensors with high speed or range resolution, high accuracy, and high reliability over long time periods. The detailed commercial device testing and research efforts of the present inventors have demonstrated that so-called Fabry-Perot lasers employed by previous researchers using diode lasers, are inadequate for practical commercial self-mixing sensors that employ diode lasers.
Accordingly, it would be advantageous to provide the means whereby lasers employing the self-mixing principle may be used to form the basis of practical, reliable, and commercially viable sensors,
It would be further advantageous to improve the dynamic range of diode-laser based self- mixing sensors and to prevent EMH from causing ambiguous measurements or measurements with decreased accuracy.
It would be further advantageous to achieve significantly higher sensitivity as well as variable attenuation, compared with the simplest form of the sensors discussed in the prior art previously.
It would be further advantageous to increase the detectability of weakly scattering objects, such as objects with low reflectivity, or small particles.
It would be further advantageous to enhance diode laser sensor operation at long ranges, without requiring the complications of optical spectral analysis discussed previously.
It would be further advantageous to achieve highly accurate speed measurements without requiring that the sensor is accurately aligned in angle to the target.
It would be further advantageous to obtain range and/or Doppler information from multiple spatial locations without the use of photo-detector arrays, as in the previous devices.
It would be further advantageous to design sensors that are highly reliable and highly manufacturable.